FLUYIATILE SHELLS 



OP 



NORTH AMERICA. 



PART I. 



Family RISSOIDAE. 



[Lingual membrane witli the inner lateral teeth very 

 broad, the apices incurved, lobed ; outer laterals dissimilar, 

 all with denticulated apices. Eostrum more or less adnate, 

 below, to the forepart of the foot ; tentacles setaceous, with 

 the eyes on bulgings at their outer bases ; neck-lobes none. 

 Foot angulated in front, acuminate behind ; operculigerous 

 lobe, with developed lateral expansions, and usually fur- 

 nished with a caudal, tentacular filament. Operculum horny, 

 sub-spiral. 



Shell generally white, spiral, more or less turreted ; aper- 

 ture usually simple in front. — S. and A. Ad.] 



Al^lIVICOIiA, Gould & Haldeman. 



Animal with the head proboscidiform, rostrum subbifid at the 

 extremity, and extending beyond the foot ; mouth 

 a longitudinal slit upon the inferior surface ; ten- 

 tacles setaceous, of equal length ; eyes at the pos- 

 terior external base, not pedunculate : foot sub- 

 ovate or lengthened, truncate anteriorly, the angles 

 capable of being turned outwards as in Valvata, Animal of Am- 



. . , , nicola decisa. 



but not to so great an extent ; and it is incapable 

 of the extension beyond the rostrum observable in Paludina. 

 Shell short or lengthened conic, thin in texture, composed of 



( 1) 



